In my experience, this isn't actually an error. It's more of a "Hey! I'm usually pretty busy, but I've got nothing to do right now...you should take a look!" or it's in the middle of a really large batch of replication commands (and it's just taking a long time to commit)....it could also be that there is a process blocking the replication spid that's currently running (very common).

Did you see any records in the [distribution].[dbo].[MSrepl_errors] table? If there was an actual error, it should be logged there or in your SQL Server Error logs...

You can change the heartbeat interval to be longer if you see this often (it just changes how frequently the check occurs).