Marathon dilemma resolved

The perinatalogist wrote me the following note in response to my marathon question:
'I think your points are all valid.
Even though there are accounts of pregnant women running and doing well, we don't have the denominator- Pregnant woman who ran and had trouble. So I would say the risk is theoretical but within the realm of standard counselling that we give pregnant women - that exercise is beneficial but we usually don't recommend to exercise for more than one hour at a time. Usually I say don't exercise for more than 30 minutes without resting for 10. Don't put yourself in a compromised state - ( for example Don't get dehyrated or cause such a build up of acid that it initiates uterine contractions.)
I think if you said i was going to run for 20 minutes , then walk for 20 minutes the run for 20 then walk for 20 that would not be completely unreasonable and I think you could still finish within the marathom allotted time ( is it 6 hours is allowed to to officially finish?) , but ultimately we would feel terrible if you went, participated and ended up getting sick or going into preterm labor.'

So I decided to still go but take it really easy. This will be so different to any other marathon. I'll try the walk/running to ensure I don't overheat. Boston here I come!