I have this year had to have a knee op and the surgeon has suggested that i stop my previous running training efforts of going out 6-7 x a week, and only run on alternate days, a max of 3 x week. To substitute for that loss of running, I have started cycling.
I go out for individual rides which are probably consistently harder as you are always pushing the wind, and I go out on longer group rides which are around the same average speed but for much lnger distances (as you do a fair amount of drafting).
In cycling, your body weight is supported, so this makes a difference in terms of PE but also how easy it is to get the HR up to the higher levels on a bike. Unless I am really sprinting up a sustained steep hill, I can't get close to the HRs I can sustain while running. Maybe that would change a little bit on a turbo-trainer, but I still expect the cycling to be relatively simpler.
However, there is no impact on the legs from cycling, so you can perhaps sustain much longer bouts of training at slightly lower cardio intensity - your running will certainly benefit from higher training volume and this should lead to an increase in your lactic threshold. Good luck.