Running is hard. I cycled for 20 years including plenty of racing (never very good) with occasional running thrown in during the winter. Three years ago I got the running bug and essentially quit cycling until recently. It took me many - like 15 - runs before it stopped feeling like torture. I'm 5'10ish 150 lbs and was totally in share from cycling and lifting. The only thing I wanted to do during my first few runs was stop. I can't imagine what that would be like if I had an additional 30 lbs on my frame. Highly unlikely I'd make it through the initial torture stage.
I did the whole marathon thing (once - normal human 3:20 time) and plenty of 5k/10ks. Once I broke through the torture part, the "runner's high" allowed me to zone out and put in some miles. "High" is fairly laughable; more like a little more pain tolerant. The huge - HUGE - downside to running as a 40-something y/o is the constant threat of injuries. Never had anything big, but plenty of mini sprains and pulls that make it really hard to move the mileage up if you want to get faster. I now mix the two and they blend well together, but in no way optimize each other.
Getting back on the bike after not riding for a year (and few rides in the prior couple) was easy. My quads burned on the climbs for the first couple of rides, but I didn't feel slow (probably was) or tortuous.