Use only your infinite incubator for 2km eggs
It's not Team Rocket science; it's just math. Let's say you have another incubator besides your infinite one. You can either hatch three eggs in the limited one before it breaks, walking a total of 6km. OR, you can put a 10km egg in that thing, and walk 30km before it breaks.
Now, let's say you put a 10km egg in the temporary incubator. In that same 10km, you could hatch five other eggs in your unlimited incubator.
Again, it's simple advice, but if the egg incubator ain't broke... well, I guess you can't really fix it anyway. You'd just have to buy a new one. But you get the idea.