Absolutely. The group ride can provide most or even all of the hill work and intervals. The purpose of that Z3 cross trainer time is to stress you and get you over that long period where nothing much happens. Then you gradually move into all bike time with pedaling drills and longer Z2 rides, then adding some Z3 midweek, then some midweek LT intervals if you're up to it. The main thing is consistency. Workouts can be short. Hit the bike at 5 and have dinner when you come home. I seldom manage more than a 2 hour ride during the week. But less is OK, too. You just won't be as fast! It's all a matter of degree, with the percentage improvement dropping off rapidly as the hours increase. If you're fit, you can double your hours and only get 5% improvement. But you won't be able to do 150 mile rides in the mountains on only 5 hours of training per week. But you can with 10!