Firing with $3 million, with a catch

I'm 40 years old, I've saved $3 million (including my home value) with a full time job in the US, maxing out my tax advantaged retirement contributions over the past several years. This year I'm ready to FIRE, but I'm somewhat concerned about the composition of my assets.

Basically, out of the $3 million, ~$1 million is my home's value. Another ~$1 million is my 401(k) account, which does have some after-tax contributions. I'm 40, so I can't reasonably access my retirement assets until I turn 59 1/2. I'm left with ~$1 million to cover my years between now and when I start taking retirement withdrawals.

My concern is that for the first half of my retirement I'll have more restricted expenses than for the second half, when 401(k) kicks in. Ideally, it would be the way around, since I'm more active during my younger years.

Any recommendations on how I should approach and mitigate this problem (a problem nice to have, I acknowledge). Thank you.