Averaging 17.6mph over 28 miles is really good going. As others have said it could be that you are pushing too hard and it could be that you're sitting stiff on the bike (stiff arms). If you are low down on the bike and your neck is constantly bent upwards to see the road then that can result in neck and shoulder ache too.
You say you don't get on with cleated pedals so that may be contributing.
Your body may feel more relaxed overall if you swapped to flat pedals and trainers.
As others have said, some photo's of you sitting on the bike would help to see if there's anything obvious.
You say you don't get on with cleated pedals so that may be contributing.
Your body may feel more relaxed overall if you swapped to flat pedals and trainers.
As others have said, some photo's of you sitting on the bike would help to see if there's anything obvious.