5 weeks is too soon for bike riding, using any kind of seat, according to my Doc when I asked the same question. He said not to do it for at least 3 months, 6 months would be even better. Heck, he wouldn't let me lift a gallon of milk (5 plus lbs) for 2 omnths. Lots of things have been cut out, rearranged and shifted around inside you, and you don't need any more additional pressures, stresses and such while healing. If it were me, I'd skip the bike for at least 4 months, and keep up a really aggressive program of walking, increasing daily and weekly, adding hills and stairs when available. Now is the time to let your body heal and that will take a minimum of 3 months internally to complete.
4/19/07 PSA 7.6, referred to Urologist, recheck 6.7 06/05/07
7/11/07 Biopsy- 16 core samples, size of gland around 76 cc. Staging pT2c
7/17/07 Path report: 3 of 16 PCa, 5% involved, left lobe , GS 3/3:6.
9/24/07 (Open) Retropubic Radical Prostatectomy performed
9/26/07 Post-op Path Report: GS 3+3=6 Staging pT2c, 110gms, margins clear
10/15/07 ED- begin 25mg Viagra nightly, Fully continent
11/15/07 1st Post-surgery PSA-0.01
01/14/08 Caverject started/stopped, aching and pain , Bimix started
02/04/08 Bimix X 6 tries, .1mL, .15mL, .2mL, .3mL, .4mL, .5mL, success at .5mL
3/24/08 ED- 6 mts.-continue 50mg Viagra and pump-no change-no response