Interval training is the best way to ramp up your VO2 max, increase your race speed, and it's also #1 for health benefits. You can do it alone, but you won't do it nearly as well as you will with the group.
Remember that these sessions are for IMproving your fitness, not proving your fitness. So there's no such thing as "waiting to get in shape" before starting.
Hope to see you there! Rain or shine!