Cycling and running can both be hard - but make no mistake, the pounding on joints on running definitely makes it "harder" to cross over from cycling->running versus the reverse.
Marathons are WAY harder than century mile bike rides. It's not even close, in my book, and I've done both. That said, you can definitely bike for much longer than you can run during training - 6 hr bike rides are longish but routine for serious cyclists, whereas 3 hr+ runs are simply too long for runners for training, even at the elite level. (They do multiple shorter runs during a day to get 4-6 hrs total.)
HR definitely goes higher for running than cycling. This has been well shown.
For sure though, a good runner (sub 20min 5k) will be a good cyclist within 2-3 weeks of moderate bike training. I crossed over to cycling after running sub20min 5ks, and within 2 weeks of cycling (<150 miles total training) I could beat a lot of Cat5s and even some slower Cat4s on the hammerfest hillclimb here. It's very rare, however, for pure cyclists crossing over to running within a month to be able to remotely hang with me on the run - there was one Cat1 guy who managed it barely, and I'm definitely not a "Cat1" equivalent on the run. (Probably Cat4 runner if there was such a thing.)
In triathlons, this phenomena tends to be pretty clear. Lots of pure fast runners going into tris for injuries, and you can be sure that if they're FOP on the run, they're FOP on bike. Not so true vice-versa - lots of FOP cyclists falling to MOP or even BOP on the run.