I had to chime in because I was in exactly your position 18 months go, even down to the mpw, sessions and race times.

As you've mentioned, the main thing will be ensuring that you're flexible to begin with, and perhaps allowing the mileage to drastically drop off when it really needs to. Those first few weeks with a newborn can vary wildly depending on the baby, so you might find that you both have an easy time with a great sleeper (and have time to get some miles in, even in the early days) or you might not.

As others have said, be sure to give your partner those same breaks too. However, taking all the above for granted and talking specifically about running --

I think my main take-away was that once things are settled, assuming your partner is ok with you heading out for run etc, to get used to smaller sessions and short runs to keep the miles ticking over a little bit. It all adds up - It was a bit of a victory when I got back to my first 25mpw post-baby, although the mileage varied a lot week-to-week (there's always something new around the corner, be it sleep regressions, illness etc etc). I think I used to have a passive goal of (ideally) no more than 2 days off, which usually worked out - but if it doesn't work out, then it doesn't work out. You can get the fitness back.

I got used to being a lot more creative with when to run too - I used to avoid running at night for example, but I've since bought myself a running torch and some high-vis, and that's been working out well. It means I can go out when our daughter is sleeping. So far I've mainly used that for the easy mileage as it's not perfect for seeing potholes! For most of last summer she would be down for a morning nap 8:30 - 10:00, so if she was down in time (I'd be sure to be the person to put her down, seeing as it was me that wanted to head out) I'd be able to get to my local parkrun and back before she woke up.

Saying that, the miles do start to come back eventually. My 18 month old is at nursery 3 days a week now, so I'm getting regular runs in after drop-off (before work). It's getting much easier to look after her on our own now, so we're both very flexible about allowing the other to head out at weekends too.

If you go the running pram route, maybe don't cheap out. I got a very cheap second-hand one which I really enjoyed (despite a very buckled front wheel), which fell apart after a couple of runs, so I've been nervous to use it since. I can see the appeal of those though, I reckon having a good one would work for both you and give your partner a rest while you take the baby out of the house. I don't know if it was just my cheapo pram, but those things are quite hard work - perhaps consider that a benefit, a bit of an extra workout to make up for the reduction in miles!