I don't know the Trionium Leith Hill HM, but I do think you have plenty of time to build yourself up to a reasonably hilly HM. I've only been running since Feb and am about to complete my first HM, which I've been training in anger for for the last 3-4 months. I live in a pretty undulating area and can quite happily run a hilly 13 miles now. Just go for it!!!