The good thing about cycling is that it's not an impact activity, so you can do it when your joints are sore from running or as a recovery activity. It also isolates your big thigh muscles so is good training for uphill running. The bad thing about cycling is that it takes a long time and there are often long bits between the hills.
So, why not do hill reps on a bike? There are many answers - such as "because it's dull". Still, I've not grown tired of it yet. I'm quite lucky in that I live within a few miles of Flash - Britain's highest village. Old Flash road deserves more than the 1 arrow it gets on the OS map. The steepest bit has over 100 metres of ascent in not very much distance and I'm pleasingly tired by the end of it. I can just do it on my road bike, but the gear ratios are not really designed for hills, so I prefer it on Mrs Noel's heavier mountain bike.
Today I did four reps of it and my thighs are now pleasingly achy, despite the rest of me not feeling too tired. Hopefully I can get out for a run tomorrow. I'm sure I could do a lot more ascent if I went out for a proper bike ride. However, that would take 2-3 hours. Today's reps took less than an hour, including the time taken to get there and back.
Here's the profile from my mapping software.
I await comments:
(a) from proper cyclists, saying how I should be doing more reps, harder hills, and using my road bike, and
(b) from fell runners, saying how this type of training won't help.