I've been riding mountain bikes since 2005, but I didn't get into road bikes until 2007.
I'm not into the distance thing but I've done a bunch of metric centuries including the one I completed yesterday at 18.2MPH. I have done a 100 mile ride with a stout group. We averaged 18.9MPH. I typically ride with groups that average around 18MPH, and I do around 90% of my rides with groups. I'm trying to ride with the really fast groups (21-22MPH) with mixed success.
Improvements happened with time, weight loss and bike improvements. The two most noticable equipment improvements were moving from a 96 Cannondale to an 06 Giant full CF. Both bikes had 105 so the groupo was not a factor. The second big improvement was upgrading the wheelset on the Giant to some Mavic Ksyrium Elites.
Weight loss, I've lost 38lbs, most of it last year, but a few more pounds this year. I'm still too heavy at 182, (I'm 5'8"). I should weigh around 160 to 170. The biggest personal gain came from the weight loss. I went from someone who struggled to stay with 18MPH groups to someone who regularly does pulls and one of the more dependable riders with the 18 groups and someone who now struggles to stay with 21-22 MPH groups.
I'm lucky that because of the way I operate my cycling hobby, I'm able to buy several used, late model bikes. I have three observations about equipment:
1. A heavy bike wears you out over distance and will slow you down. By heavy I'm talking over 20lbs. I love my 01 Bianchi, but it just doesn't cut it compared to the Giant.
2. If you're a fast rider or want to be, ditch the SORA equipment. I'm not saying you need Dura-Ace, but SORA tolerances are too wide resulting in slow shifting and will hurt you especially if you ride in hilly areas.
3. Don't underestimate what a good wheelset will do for you. I wish I had upgraded earlier.