Recently I've been experimenting (for several months now) with the following on my easy days. During the run, after somewhere between 30-50 easy minutes, I do sets of pushups, abdominals, leg raises, squats (10-30 reps, depending on the moment, how I feel), with no rest before or after (I just get up and keep running). I accumulate reps to 100-200 in each exercise in total, jogging maybe 100-500m in between, and then continue the easy running to home (doing the reps this way is much harder than it would be doing them in isolation). My overall weekly volume is in the 75-90 kmw range. I take about one day off every 10 days. I'm doing this usually during 4 runs a week (so not when I do hills, run fast, or do a long run). I'm 45, about 6', 170 lbs (I was a defensive lineman in school, so I'm not built to get much thinner) at this point running mainly for fitness.
I'm finding it is helping my form (particularly arm movement and leg lift) and speed. It seems like the benefits come from running after the repetitions with the muscles tired and the heart rate up (doing these exercises raises my heart rate). It doesn't seem to hurt the recovery aspect of easy days at all (maybe even helps). My question is whether this makes sense, is it stupid, is it a good idea for some physiological reason?