You want to keep muscle mass... for what reason? For strength, or for power?
Power wise, go for sprints... they'll activate your muscles more deeply, burn is faster, and stronger, but your endurance will begin to fade. Take it in the same stride as lifting weights 800 times at 1.5% of max power, compared to lifting a weight 8 times at 90% of max power. A sprint will hit you faster, since your nuscles are being activated so quickly, be wary of pushiong yourself too hard, your mind drifts during sprints, so you can't focus directly on your breathing, or the burn... You may get injured, so short sprints is key for a beginner.
For strength, go for jogs... they'll activate slow twitchers... Go for them, stretch and compress, your muscles won't tear inside of you, they'll just get more pumped. So you won't get the RIPPED look as fast, but you will get the BUILT look.
You will almost always attain the LEAN look, when you focus only on running... as it is a practice in muscle dynamics, or aerobics, whichever is your style, increased VO2 max, all that jazz.
So what is your GOAL when you do this, we're aiming at different goals...
I aim for an undemeaning look, wirey I'd say, moderate weight, no blging muscles.. Bruce Lee... Not chuck Norris. So I used to do a lot of cardio, and I was brought down to 4%BW at 154lbs, that was just nasty on me, I enjoy being able to check my friends in a good game of hockey and not have people tell me I'm bony. So, I got into lifting, and eating AMAP, and I ballooned to 225, got the chubb going on, but I could bench more than 320 but I had horrible endurance, couldn't last more than 30 minutes in the gym without having to go for a nap
.
After those days I broke down, my mind, my soul, and my body... been searching for a good answer... I'm even at 190, have been 190 everyday on non-sick days for at least 3 years now... pot belly, flabby muscles and arms, twitchy muscles, stiff back and neck, spasms every day... I can't sit still for 30 minutes in a theater without popping a few big muscles... So I'm looking for determination...
What is it you look for in a workout? Big beach muscles? Or lean look, and being able to carry yourself, and handle yourself when the time comes?
Exercise safety, instead of intimidation...