If you're completely uprooting yourself, will you be riding with a full load of camping and cooking gear? The fitness aspects of loaded touring and randonneuring are similar, except (a) you won't ride as fast as you expect with a load, and (b) when touring you get up on the 3rd, 4th, 5th, and subsequent days and ride. Touring is more a matter of sustainable riding -- you will be riding tomorrow, after all -- while brevets are more a matter of seeing how far you can ride within one to a few days.
That said, most E-W riders seem to start the first half of May. The passes will be open when you get to the west, but it can be hot crossing Missouri and Kansas. You may be OK if you can maintain 70-80 miles per day (remember, it's got to be sustainable!). With my 50 mpd average, I've thought I might start mid-April if I were to do the TransAm again.
Pat (RUSA 8127)