RoadRaven always has good things to say. As a newbie myself, I can't really speak from experience, but I know with running and with what DH tells me about cycling, it's good to do at least one "speed workout" a week. (Which RoadRaven also suggested.) Do a shorter route than you might normally do and just go for it. Keep track of your time.

Do you sprint? If not, you might want to incorporate that into your riding. Standing and pumping hard in intervals can get your speed up. I do it at the beginning of climbs sometimes, or the middle of a climb even, after slowing down or stopping for a signal, at the bottom of a hill going into a flat when I want to keep my speed up. This works completely different muscles, so it might take a while to get used to it. The first time DH and I stood on a hill on our tandem, I could barely go five seconds and I was dying. But I'd suggest standing on the flat at first, if this is new to you, which it may not even be. I've been working on standing on my single and now I'm getting better at it on the tandem and hills. DH and I went for a tandem ride the other day and I am SO impressed with myself at how long I can stand going up hill now!

DH said that Lance Armstrong used to (or still does?) a training ride where he'd stand for a bit and sit for a bit ... throughout the entire ride. You might start with standing for 5 or 10 seconds, then sit for a minute, then stand, etc. As you are able, start adding length to your standing time.

I've yet to consider doing a group ride myself for the same reason. I did my "speed" workout today in fact. I went 12 miles, with hills, in 15.5, but I don't think that's going to cut it in a group. At least not the group I have the opportunity to ride with. It's possible I would do okay, but I'd rather work on my speed more and feel more confident before I try.

Oh ... gosh I'm just full of information! Working on hills is also a good way to make you faster on not only the hills, but on the flat. Anything that makes you work harder, makes you stronger, which makes you faster. (I get most of my riding tips from DH, who is a pretty fast rider. Came in 4th out of over 300 at a 10,400 foot elevation century recently.)