Join a race team. It's done wonders for my fitness. And don't use age as an excuse. Our best Cat 5 guy (now up in the 4's) is in his early 40's and placed 2nd and 3rd in races where the other big team in these races were college kids in their teens and early 20's. I'm the second to youngest person on our team at 28 years old.
If you've only done 600 miles of base, then yea, there is huge room for improvement.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter