The point of cycling is I feel there is 'training effect' compared to a lower intensity walking workout. Walking is better than nothing. However I get more of a boost with less exhaustion from the cycling workout than I do the long (hot) walk.
65% or 75% MHR are kind of arbitrary -- the "220-age" formula for MHR is not that accurate, and one person's thresholds are going to be different than another's. In terms of RPE (perceived exertion) or my own sense of how lactic a workouts, I feel like I have a lot of headroom before lactate threshold (LT). There is probably a value to mixing some base-training / recovery rides (<=65%) and some more intense rides (65% - 80%) that flirt with LT and have a real training effect.
Be prepared for appetite to increase.
I'm also doing some upper body weight lifting. The theory behind skipping leg day is that I'm already putting 2 or 3x the normal load on my lower extremities just by walking around at 285lbs.
Note to self: when screen shot-ing polar flow graphs, it's useful to anisotropically scale the image in the vertical direction and then add in the 65% and 75% MHR lines. The 70% line is already represented by the blue/green transition. The scaling gives you a little more resolution on the graph line - values between 60-70% only occupy an inch of screen real estate in the original viz on polar flow.
