Rest before a Marathon

Hi, i am just after some advice. I am keen to do the Bournemouth marathon in October but 3 weeks before I will be cycling from London to Paris. This is a 3 day event and the pace is quite relaxed, this will also be the 4th time I would have done this so to honest I don't do much bike training for it anymore just as long as I am fit I am normally ok. Do you think 3 weeks is enough recovery time between events or is it a risk? I did my first marathon last year so I am not a beginner but no way an elite. Thanks in advance