On the Treadmill, literally and figuratively

yesterday no riding- but we went to the Y after work and I did 20 minutes on the treadmill and did one circuit of Nautilus (no leg machines)
This morning on the windtrainer:
20:00 warmup
10:00 FT+10%
10:00 R
10:00 FT+10%
10:00 Cool down
Tonight: Pilates!
In spite of a careful diet with careful portions, I'm adding some muscle mass, but what better thing to replace fat with, than lean muscle? Keeping the Nautilus reps at 20-25 is probably a good idea. I don't understand how there is any health benefit (besides getting ripped) to lifting the whole stack of weights for 6-8 reps, yet I see a lot of older, taller, yet pear shaped guys doing this. I guess it's possible to be strong as an ox AND out of shape.