Great to hear you are feeling good post half and gearing up for other ventures this spring. You should plan a marathon. To do it right, you need 6 mos of specific training and at least a year of running and being active . You have the first 6 mos in nearly–now you could pick a race this winter or coming spring and start to get ready. The marathon is a totally different animal than the half. You can’t fudge on the training at all. You have to be super fit just to get through it and your long runs in or 20 miles or more. I’m excited for you that the half has got you thinking about what is possible with more training and time on your legs.