I get this as well and I agree it as really frustrating. Somedays I have not been able to do hardly anything. I tried to clean out the fish tank a couple of weeks ago and didn't have the strength or energy to do it. I find the longer I am on the citalopram the better it gets. I had a setback this weekend and it came straight back after 2 weeks of not really having it at all.
Getting on your bike sounds like a great idea. I can't ride my bike at the mo for other reasons but I am dying to get back on it. I used to ride a good 40-50 miles a week before march and it is killing me not being able to 'cos it is the one form of exercise I love. Me, My bike, My MP3 player and some country lanes!!! It is a great way of building up strength in your legs and helps with stamina and general fitness as well. If it is just your legs that feel weak and you are not getting dizzy etc you should be fine. Just ride a couple of miles to start with and build it up. I would really give it a go, it will work a lot faster then walking.