I've said it before, but IMO if you're not sure about stepping up from one distance to another, it can be a good idea to enter an event of the lower distance that's about 30-60km from home, and plan to ride there and back. Enter it as an ECE if you want and you're adding >100k (and benefit from a lower min speed for the overall of extra+nominal event distance), or simply ride it - no real time pressure, and a real boost to confidence once you've done it, because if you've ridden (say) 35+205+35km, a 300km event is only 10% extra.
(Or alternatively, just say sod it, and enter. Chances are you'll ride a 300 faster than 1.5x a 200 anyway, because of groups forming and greater control spacing, plus this *is* described as a fast route ...)