You can playback records on your laptop computer, at your buddies’ place, in earphones, on your CD player and even in your car. Make notes. If it fits the playback system you’re using, it’s worthy of efforts and time spent. If anything goes wrong, guess what’s bugging you and find out if it corresponds to what you hear in similar systems.
Brief summary: If mastering keeps going long and isn’t easy, something may need fixing in the mix. Try different options with the equalizer to find out what is coming about. After that you should decide what to do. You will probably have to request revision from your cheap online mixing and mastering services. If it’s necessary, you can use compression as an 
instrument to drive the groove of the record. You can also enhance loudness but without depriving listeners of their pleasure.
