Looking for magic answers from older riders (those with slower recuperative powers). I am quite sure the answer is to practice going faster, probably via interval training, just like it is (I think) for younger riders. But I am often surprised so I ask.
One advantage of aging is less worry about acting/looking/sounding stupid. My kids insure I never forget I already do! I am 67. I used to be somewhat hard core - I was a runner, my 5 marathons (ages 38-40) were 2:51-3:00.
Today I rode with a group of twenty riders, mostly over 50. It was a 40 mile ride with an average ascent of 40' per mile. My average speed was 15.3 MPH (all average speeds are "while moving"), my average heart rate 127 (LTHR = 143 [I think], max ever seen riding = 160), average cadence = 88.
I simply could not keep up with the faster riders on the flats. In my almost 3 years of cycling, I have had very few rides with an average speed over 16 MPH. I did 15.3 on both centuries my first year (I was riding a LOT - almost 5K miles my first 7 months). Almost all 16-17 MPH rides were shorter rides (20-30 miles) while I was making a serious effort to go faster.
Weight - I have put back on 25 of the 40# I had lost last year in 5 months (a bad side effect of breast cancer) so I am now 220# (3.0# / inch of height). When I was around 200# last year, I did find I was faster. Hills simply did not affect me as much. I expect (hope might be a better word) to get my weight back down to 200, but I doubt I will ever get it down to 185 (2.5# per inch).
As an aside, I was lucky and the chemo, surgery, radiation did not screw me up very much. I just finished radiation ten days ago, but I was riding (although less often) throughout treatment and my riding was not affected very much (other than a much higher HR during chemo). The only good side effect I hoped for was to lose some weight, but the chemo never diminished my appetite. And I think (or maybe it's just an excuse) dealing with stage 2 cancer triggered the "eat and live" response.