It sounds like you are having a challenging time with your back symptoms and certainly walking should be an easy task that does not lead to aching.
There can be many different reasons for back pain and it maybe worthwhile getting your back assessed prior to undertaking an exercise program. This might elicit the areas that require work and certainly if there are any points of concern, these can also be addressed.
If you are keen to look into the strengthening as a first line, then options include seeing a Sports Physician/Physiotherapist or a personal trainer with experience in dealing with back pain.
The important elements to consider are:
- Managing weight - so cardiovascular exercises are useful
- Strengthening the “core” muscles - this has become a big buzz word in strength and conditioning, but essentially it means strengthening the muscles around your midriff and re-learning how to appropriately engage them. There are many different exercises that have been developed to enable this and simple examples are the plank (and variations to this) and breathing exercises.
- Focussed exercises for areas of deficiency
There are many online programs that are also available, but please take case as doing them without supervision or if done incorrectly, it can lead to injury and further symptoms.
I hope this helps.