Good work on the training and goals
I'm a vegan ultra runner - done over 25 ultras (got a 40 miler in 2 weeks), iron men and in 2010 I did a double ironman, so there's no issue with not being able to do it as a vegan (you just have to be stubborn and stupid, but that's a different matter
So - I don't do anything special diet wise... I can always get the calories (judging by my hard to get rid of spare tyre around my waist). When I'm doing long long miles - I just make sure I graze a lot throughout the day. Like Konstantin says .... make sure you eat enough!
It's pretty much the same as anyone else will say for vegan training - lots of whole grains, pulses, legumes, fruits. I eat dried mango a lot while doing ultras, but I also eat sports gels (currently liking torq) and my diet is by no means perfect - I get back from a 25 miler and eat some crap.
You're not going to lack calories as a vegan, but you might need to plan your food a bit better so you don't get stuck somewhere and feel the need to relapse. I'm lazy, so I have some goto foods which I always have available (be that dried mango, or houmous, avocado & bloomer, or an easy raw pressed bar like these http://www.naturalbalancefoods.co.uk
) so I never get too caught out.
Also have a look at what Scott Jurek can do http://www.scottjurek.com/blog/
Good luck with all the training and hope you can make the switch
edit : here's Sunday's food from a 20 mile run..
3 weetabix & almond milk
torq gel / some dried mango while running / high 5 sports drink
hazelnut milk smoothie with banana, raw kale, blueberries, quinoa flakes, seeds, hemp oil, walnuts, maybe a scoop of hemp protein powder
Lentil salad, houmous & bread
some marmite rice cakes
brown pasta & fake mince with veg
anything else I want if the calories are still low
(I do rough count on calories on hard days to check I'm getting enough)