Fugly and Colossus both gave excellent pieces of advice. My dad had been experiencing similar symptoms (he just turned 52) and found out he has hypogonadism (low test levels). If you haven't had a thyroid workup done, checking TRH, TSH, T3, T4, TBG, and free vs bound T3 and T4 levels could let you know if you're having a thyroid problem, which would cause a drop in your metabolic rate.
That's a lot of stuff for thyroid and your doctor (or your insurance company) may not want to spend the cash to have all that checked, but the thyroid system is complex.
Another thing. My father and I have had stomach problems our entire lives. We don't have Chron's disease or Colitis, but both of us have recurring peptic ulcers and generally sensitive stomachs. It's particularly exacerbated during periods of high stress. When my gallbladder went out on me about 3 years ago, I was constantly sick to my stomach to the point my gastroenterologist thought I may have one of those problems before diagnosing my gallbladder. There were a few things that really helped me out during this time:
1. A good probiotic. Not that Dannon crap that's like liquid yogurt, but something that will provide you with cultures on the order of several billion rather than several million. I found that Align and Culturelle worked well for me. My dad (he's a doctor) get's free samples of Align so I take that now since it's free and a lot of his patients with problems like that find it helps them quite a bit.
2. Ginger. Ginger was/is a Godsend. It activates the same serotonin receptors in your brain that prescription medication Zofran does (5-HT3) and has always been really good at calming my stomach down. You can either buy capsules of ground ginger or just buy the root and boil it down to make a tea. I find that making a strong ginger tea is more effective than the pills, but both work fairly well. Ginger also promotes the production of bile salts in the liver and their release from the gallbladder, which should help you digest food better, particularly lipids. It also acts as a mild gastroprokinetic.
3. Papaya/other digestive enzymes. Papaya has been used for centuries in south America to treat stomach problems. You can buy papaya enzyme pills (BTW, you can get all this stuff in the vitamin section of Walmart) that contain a certain amount of raw Papaya and the active digestive enzyme in papaya Papaine. I also take a multivitamin from DaVinci Labs called Spectra Man (you have to buy this one online) that contains a complement of digestive enzymes in it... amylase, protease, lipase, hemicellulase, and lactase. One the biochemical side of your medical condition, one of the reasons you may have difficulty tolerating certain foods is because your body is failing to express or properly express the genes that encode for certain digestive enzymes. Sometimes, an effective treatment for people who are lactose intolerant is to supplement with lactase enzymes every time they consume a lactose containing product.
4. Medication. If you have been diagnosed with a gastro problem, this has already probably come up. On the prescription side of things, Phenergan is an antihistamine that is very good at calming your stomach down. It's like a stronger version of Benadryl. Zofran is very good at calming the stomach down and stopping vomiting. Compazine is another classic anti-nauseal drug, but I do not think its as efficacious as Phenergan and Zofran, unless your nausea is due to opiate use. Reglan may be useful because it is an effective antinauseal drug and makes things go through your system faster... but it has some neurological side effects that may be permanent.
On the OTC side, any formulation of Meclizine will be a somewhat less effective choice than Phenergan, but less sedating. I believe this is sold under the name Bonine. Dramamine is also ok, but it's more sedating. Benadryl is probably the least effective OTC medication. Also, if your stomach lining or intestinal tract is irritated rather than you having an intolerance to something you ate, don't underestimate the ability of good old peptobismol.
5. Making a tea from Fennel Seed is good for gas related problems.
Finally, one axiom my GI doctor and my dad swear by is a lean colon is a happy colon. If you can manage it, ramping your exercise up may very well help you. Walking is good, ideally you want to do something that's going to make you sweat and stress your heart. Since you've had a back injury, swimming may be a great exercise for you since it is such low impact.
The GI side of things is very important because without a good diet, it will be difficult to maintain a decent level of energy. If your stomach problems are inhibiting you from consuming what you need, that should probably be addressed first.
After a good diet, exercise will also do a lot to stimulate your energy level. Studies show that after 2 weeks of consistent aerobic exercise, the mitochondria in your cells double. Remember, the mitochondria are the molecules in your body responsible for the synthesis of ATP, which is one of your bodies main sources of energy.
I would recommend enhancing the aerobic exercise you already do and make sure you do it 3x per week. You want your heart rate up, and if you're not sweating, you're not working hard enough.
I would also recommend speaking with your doctor and determining an appropriate strength training program. FYI, an orthopedic doctor I know said men over the age of 50 are not supposed to bench press because of the risk it poses to their bone structure. He recommends (and I know the guys here will probably sigh at this suggestions) focusing your exercising on machines rather than free weights. The more fat you lose and lean muscle mass you develop, the higher your metabolism will go naturally. The more lean muscle mass you have, the more testosterone your body will have to produce. Both of these will facilitate higher levels of energy.
I hope this helps!
