20k distance still falls into the same energy balance as a 10k really, just a little bit more conservative.
Assuming you are doing it in under 2 hours anyways.
So you have really already had a good base for the job at hand.
2 hours is long enough that in some conditions you may have to start considering your hydration and realistically, over that distance you your best bet would be an Gatorade type drink.
So get used to carrying and drinking on the fly.
You won't need much, but the time out suggests that there will be a performance gain in doing so.
Try and get out for the distance every now and again just so you know of any discomfort issues that may arise and to get a feel for the slightly slower pacing required if it is a slog.
But otherwise, you are already doing just fine.
If you really want to get keen, you can get some structured intervals aimed at that distance and play around with a taper that suits you, to put in action, so you arrive on race day fresh.
(If you were really ,really keen, you would have a H/R monitor and know intimately your anaerobic threshold and the pacing required to complete the particular distance.)
The extra distance also puts more emphasis on learning to get the water to help you, practice taking on some smaller slower runs instead of busting through them, and try and save a little energy early on in the race.
I honestly don't think you will have any problems with the extra distance.
Just get out there and do it a couple of times in training and get you confidence up.
Don't forget to stretch......
Disclaimer:
I know jack shit.
Any coaching quals I have expired 5 to 10 years ago.
I haven't raced for over 5 years.
Everybody on this forum that never bothers to post probably knows more than me.
Don't trust some knob on the interwebs for anything important to you.
Oh, and buy a faster boat, as he who has the most toys wins.
Follow the path of the independent thinker. Expose your ideas to the dangers of controversy. Speak your mind and fear less the label of 'crackpot' than the stigma of conformity. And on issues that are important to you, stand up and be counted at any cost.--- Thomas J. Watson