I'm an old fart coming off a decade of setbacks, and am now alternating 75 minutes each of stationary bike days and running days. For the first time in decades I feel I'm getting stronger without all the little aches and pains, much less any hint of injuries.
In part I am inspired by a younger masters guy I am coaching who has incredible hill running ability (my weak point). He was a Cat 1 cyclist for 30 years and still commutes100+ miles/week on his bike. He is effortless on the steepest hills. On his running days it is almost all quality and he is already easily winning his age group in 5Ks after only a few months of aerobic + tempo + short hills (no intervals), incredibly tough racer but no sign of injuries or setbacks.