You won't have as much flex in your ankle once it's fused as you did before but should hopefully have less pain. The pain when walking makes sense above due to the loss of movement in the ankle yet you don't really need to bend and flex the ankle much when cycling so as said above cycling should be back to normal after surgery and as tempting as it is don't go too far too quick. Your muscles and ligaments will be needing strengthened back up again after surgery as you'll have been less mobile whilst recovery so little and often is ideal.
Also, as much as they're handy please try and stay away from the supports you can get as much as possible. Without trying to contradict myself by all means wear one when your walking, cycling etc if you really need to but ONLY if you really need to and take it off as soon as you stop to rest. Reason being is that if you wear it all the time the muscles, ligaments and tendons around the ankle relax and lose tone as they don't need to support the joint because the support your using is doing the job for them if you see what I mean.
Mounting and unmounting as you've said could be an issue at least for a while after surgery as you won't be able to put weight on the ankle for some time.
I've had a bad right achillies tendon since last September, running (which I hate anyway) at one of my gym classes. Felt it at the time but was able to continue so didn't think it was that bad - until the next day when I could hardly walk! And it's still causing me problems now and cannot walk on it at all first thing in the morning or if I get up from sitting for a while however I can cycle until my heart's content!
Bone heals better than tendons and ligaments as they just tear and take alot longer than a broken bone to heal so please take care of these and they'll take care of your fused ankle.
Lastly remember RICE = Rest Ice Compression Elevation especially when been doing any activity and / or if you feel pain.
Sorry if i've bored you, it's my job