Yes, while I ran my first 2 marathons recently (end September and early November), I won't be able to maintain that level of fitness for an entire year. Especially now that I haven't been going above HM due to the weather/suffering from a cold. I think autumn is probably the best time to run a marathon compared to spring. Not too warm, not too cold and the bulk of training is over the summer months. For a spring marathon, most of the training would be in winter. Let's see what happens to my London Marathon ballot entry. Will have to choose one as obviously I can't do it on 2 consecutive Sundays lol. Might also consider an ultra next year.