there you go! The first couple times are the hardest, I agree. Once you start building up a bit, it gets really fun! I went from having to take a rest on my 10 mile commute both in the morning and afternoon to lamenting that it is not long enough! I am on vacation right now in NH, and brought my bike so that I would not miss out on riding.
Good luck on the knee surgery too! I know that biking is waaaaay better on my knees than running was. TBH I can't take the pounding on my knees day after day-this was a major factor in me plumping up a bit. For me, biking has the reverse effect-my knees feel better! Have you spoken with your Dr. to make sure that cycling is fine for you? Also, albeit stationary probably in the beginning, cycling could be a part of your PT in recovery.