In relation to the OP, just ride your bike more, you are not at the point where you need to be concerning yourself with structured training and most certainly do not need to be consuming dietary supplements!
Just to put things into context:
A 20 minute ride on a fast course (with traffic) is not at all rare amongst amateurs riding on the open road.
In the last fast course 10 mile TT (fastest rider category) I raced, 17 people did rides of >30mph. 3 riders did sub-19 minute rides. The fastest time was 18:14! I suffered like a dog with conditions and lack of form and ended up well down in the standings with a >28mph ride!
endoman on here has done a few rides of around 20:30 this year on that fast course. My time of 21:22 on said fast course was on a windy day and being 20W down on my usual average 10 mile TT power due to a dip in form after a week away eating lots of Chinese food and an overdose on Worthers Originals. So would be doing under 21 minutes when on form!
I would expect to be able to go >30 mph for a 10 mile TT with a couple more seasons experience! Endoman most certainly has the potential to be doing so!
I would suggest that your chosen course is either inherently very slow or your clubs elite are not all that elite. I am doing short 22 minute rides (~27 mph) on crappy Cheshire courses and I am nowhere close to our clubs elite (about a minute and a half slower than them)!
24mph is a 25 minute ride, which is rather timid unless the course is a proper dog! I have done faster rides than that when just looking over a course!