I'm not exactly the same since I wasn't completely new to cycling, and I had spinning and shifting in muscle memory, but my fitness level was probably comparable to yours.

Three years ago I returned to cycling after 12 years of not riding a road bike.

At the time I was gym-fit - aerobics almost every day, heavy strength training, a bit of kayaking, a little bit of running but not near what I do now.

I went to a retreat where we did three consecutive 45-mile days on demo bikes in moderately hilly terrain (Napa - Sonoma).

It wasn't the slightest problem for me. No soreness that I remember, no excess fatigue (well, the whole retreat was exhausting and exhilarating, but not just from the cycling!).

Gym to 45 miles in 0 days. You can do it (if you don't try to go nuts on the speed).