Hi

The WKO training peaks software is very good - but you can also keep a decent log of activity with Golden Cheetah (freeware).

As far as training plans go, if you are reasonably fit the first thing that you would want to work on is base aerobic fitness. That can be done with lots of extended riding in the broad aerobic zone, or by building up intervals averaging the top end of your aerobic zone (which sounds like it may be appropriate for you).

So to set your targets, you need to ascertain either your lactate threshold HR or max HR (plenty of info available on these on the web). Maximizing your aerobic capacity will give you a better chance of hanging with the group in your first races - after which you can begin working on building other capabilities (VO2Max training, anaerobic power, neuromuscular bursting).

If you find you do OK with the solo suffering of intervals, you may like to look into tying a power meter to your Garmin in the future - where you can tailor your workouts much more and get more useful data.