Deadwood to Englewood and back to Deadwood, two-day round trip.

I've ridden 400 miles in four days, with 200 of those 400 miles on gravel.
Is late April or early May to early in the season to ride the Mickelson trail?

Well, personally, I wouldn't try it that early in the season.

March is normally the snowiest month in the Black Hills. The average highs and lows in Deadwood and Hill City even the first week of May are 60 and 30. If there is a lot of show in March, I'm guessing it may not be melted by early May. Even if it is, much of the trail will be sloppy and you won't ride the 100 miles each day as fast as you normally would. If there is still considerable snowpack, you'd probably be walking your bike in some places. Even without snow, riding at 30 degrees early in the morning or late in the day isn't my preference. I'd rather ride at 100 degrees than 30! But that's just me.

The first "Trail Trek" in 2025 isn't until June 20-22. That gives the organizers time to get the trail in order. For this year, I'm planning a late May or early June up and down 2-day trip.

Now, I'm not saying you couldn't or shouldn't try it in late April or early May. I'm just saying the odds are that it would be difficult. Whenever you decide to do the ride, all the best.

P.S. Locals might be able to give you a better answer than I have. But this is what I'd tell anyone I know. Wait until late May or early June to enjoy the beautiful scenery along the trail. You may never pass that way again!