FWIW, IMO...
My first year of road cycling, racing the CCCC crits I started placing in D grade straight away and moved up to C grade which didnt seem much faster, maybe just more consistent pace.
Weekly KM total was about 100KM including the crit.

Track season this year I had about 2 months of 200+KM weeks then started track training and follishly stopped riding the road bike (got the track bug!)
Started winning D, then C, then B grade races over about 6 weeks before body gave up.

Anyway, the point id, it deemed like it doesnt take much to get improvements to a level (still crap) but sustaining that with no real base when you stop training is the real trick.

e.g. Jock has a solid training plan and base, went from abusin me in D/C grade to racing hard in A grade and getting results pretty quick. Least it seemed that way to me, I wasnt the one in Akuna bay at some ungodly hour on the weekend!

e.g. Climbo has a base fitness from some other planet and turns up setting the A grade pace with little to no training.