srw
It's a bit more complicated than that...
Marylebone to HPC is easy - down Seymour Place to Marble Arch, through the arch and use the pedestrian crossings to get across to Speaker's Corner, then down the Hyde Park bike path to the end. Again, pedestrian crossings to HPC.
Once you're back at Victoria, either ride up to HPC and do the same route in reverse or find a circle line tube back to Edgware Road, which is a very short hop from Marylebone.
Unfortunately I'm not going to be on this one otherwise I'd offer to meet you at Marylebone. I don't know if @McWobble is on it - he also arrives into Marylebone.
Once you're back at Victoria, either ride up to HPC and do the same route in reverse or find a circle line tube back to Edgware Road, which is a very short hop from Marylebone.
Unfortunately I'm not going to be on this one otherwise I'd offer to meet you at Marylebone. I don't know if @McWobble is on it - he also arrives into Marylebone.