I think some of the more experienced folk here will probably have better advice than me, so hopefully one of them will step in if I'm steering you wrong. But, since you know you won't be able to ride as long, maybe try upping the difficulty level of the shorter rides that you'll be able to put in, and include some interval training?
You say you are doing moderate climbing now in your longer rides, so maybe find some steeper climbs to work on, or do hill repeats where you ride up something, then down, then back up it again. Another alternative would be to put in intervals during one of your shorter rides where you're just really pushing it speed-wise, then drop back down (not to a rest, just back to normal).
The bursts of high intensity effort should help train your muscles to recover quickly from exertion, which will help your endurance even though you haven't been able to put in as much time on the bike.