Try changing your trainers - the ones you are wearing may be forcing your foot to land 'oddly' without you realising it - this would put slight pressure on your ankle and your shin.
Try taking two ibuprofen before you set out. Leave home 10 minutes earlier and start your walk quite slowly and build up the speed gradually.
Whilst sitting down do a simple exercise as often as you can - extend your legs in front of you and flex your feet so they point to the ceiling - hold for a count of ten, now straighten and hold for ten.
Repeat 10 times.
This will increase your flexibility and will help if you do it often enough.