they will not go rancid. the yeast has taken over in the beer (hopefully), there shouldn't be a whole lot else active that can get the rotting process going.
how diligent were you in getting the peel only, and avoiding the pith (aka the white stuff under the external peel)? the pith can be quite astringent. as long as you don't have too much of that, you'll be fine.
in fact, the chances of things going wrong are pretty slim based on my limited experience. i did exactly as you did on my first batch. upon bottling the beer smelled really strongly of peel, i was afraid that i had ruined the batch by leaving the rind in primary. the beer turned out great.
for future reference, you should strain out the peel while going from kettle to primary.
and the peels don't need to go into secondary, necessarily. adding peel to the last 5-10 minutes of the boil is the traditional way of getting the flavor into a wheat beer.