Fire regulations when not camping

I have another in my seemingly endless series of dumb (or at least fairly pointless) questions. I understand the 150' rule for camping and do my best to abide by it, though I don't actually measure the distance. My question is about old fireplaces that are too close to a trail or lake to be legal campsites. The one I have in mind in particular at the moment is at the lake end of the Watch Hill trail by Indian Lake, which is sort of a fireplace with a chimney on top, all made from mortared stone. It is very close to the trail, though the trail does not seem to get a lot of use, and it's also only about 50 feet from the lake bed/edge of the woods, as I recall. Is it legal to make a fire in one of these old fireplaces, providing that the fire is not left unattended and is completely extinguishinged at the end, or are fires only legal in places where one can also legally camp? I don't usually make fires but there are times when it is nice to have one for a little while and it would seem that a purpose-built fireplace would be safer in some ways than a newly made fire ring. Another thing I could do if this is not legal is to go to a picnic area to make a fire, as I believe they have legal spots with helpful little fire-containing doodads. Any advice will be very much appreciated.
Zach