You have two choices, it seems to me. Either enlarge the strips so that your mower will go along them, or buy another, smaller mower of the same type as your large one - i.e. if you have a rotary mower, get a smaller version, and steer clear of the 'floating' type, or vice-versa.

Depending on the length, if not too long, I would try it with what we call a whipper snipper. Typically this is used to trim back borders. With enough practice, you will eventually get the knack of keeping it at the same level for a neat appearance. If the area is too long, this may prove to be too difficult.

I think a strimmer would be difficult and shave some areas which would end up bare or mossy. I agree with Spritz widen or buy a smaller mower.