Oh forgot to add
Will do a few hundred sit ups.

Eavesy you mentioned your back before. Are your bars still low mate. Are you reaching too far.

If your back is aching you may be trying to walk before you can run. If your trying to blast along you can put pressure on your lower back if your body isnt adjusted to the rigours of riding for long distances.
It can happen easily if your out the saddle a lot on the climbs.

Try doing a ride a lot easier and not push to hard. If your back is okay after that it may be a case ofw just building it up steady mate