Something treaty - doesn't have to be related to running. She might be sick of all things running after all that training! She's probably been cutting back on nice things recently too.
Things I'd have been delighted to receive would have been things like celebratory fizz, voucher for coffee and cake, Bloom & Wild flowers...
Or take her out for a drink (cocktails?) or lunch and let her bore you stupid telling you all about it! I didn't say anything to anyone who didn't ask, but would wax lyrical about it to anyone who did! It's such an amazing experience (and I'd run marathons before) that I was on such a high, I just wanted to relive it over and over.
The thing that meant most to me was crossing the finish line, opening up my phone and seeing all the many, many messages of congratulations. SO MANY people had been tracking me as I ran, so they all knew I'd finished. I hadn't even collected my bag and my phone was just buzzing. It was the best feeling, knowing that people had been following me, rooting for me, and had taken the time to send a message in that moment.
So do track her, and congratulate her when she's done - that may well mean more than any physical gift.
She's going to have the BEST day - wish her luck from me, some random on the internet!