YA definitely NBU and thats as someone who has run a marathon (whilst my H looked after our 3 children).
BUT, we discussed it first, thought about what the training plan would be, and he was supportive.
For everyone saying it doesn't take over your life or impact family laugh - you have to be kidding!!
If you get up early in the morning to run, and then do a long day at work, you're unlikely to be the life and soul of the party by 9pm! Similarly if you go out running when the children are in bed, you're not spending time with your spouse in your "child free" time. If the OP has been on her own all day with children, then maybe she wants to spend time with her husband in the evening. Completely reasonably.
I followed a 4 month plan for the marathon - by the 3rd of 4th week, the weekend run was more than an hour. And that's just the running -not the warm up time (making sure you've eaten, got drinks organised, got changed) or afterwards (shower etc). Its easily 2-3 hours out of a Saturday or Sunday for the long weekend runs early into a training programme.
And that's before the charity element. Some charities DO make you commit to a minimum amount and pay it yourself if you don't raise that total, and also fundraising is time consuming.
Its not unreasonable for your H to suggest it, but it needs to be a family decision. Most families have limited family time. Your H unilaterally deciding he's going to take up a chunk of that family time for his own activities, without any prior discussion, is completely wrong. YADNBU.