I would like to share my personal experience on the topic.

When I was 16-18 I used to run every morning - 6-14km EVERY morning - outside, along the road that climbs the mountain. So - 8km walk is really nothing!

About swimming - I have been swimming for 2 years, 2-3 times weekly, every time 1-1.2km for about 1h - so this is really possible and achievable by anyone with a strong will.

As a whole I think you should understand that strength is not a matter of physical body, but mostly a matter of focus. If you are focused inside yourself - then you have strength to do anything, and most important - kriya yoga to the extent of khechari mudra.

A strong person never feels split inside in the way "should i join this course or not" and never feels need to fight. Fight is a matter of inside contradiction and one who needs it - has a deep inner conflict that MUST be solved first.

Just after that, when you really feel the deep peace inside - the Goal will come to you and after that - the method to achieve it. Thai Chi is a science of harmony, not a science for single chakra - as are all spiritual teachings in a way or another. There is a chapter on this site that explains that different ways are leading to God. But whatever path you choose - be sure it's right for you (you should feel that, not think of it) and don't change it.

Some years ago I was in a similar situation - searching for paths, teachers etc... This really leads to nothing but problems and pain.

Forget about anything. Relax. Feel yourself - and then you will know what to do. Don't be afraid to stop for a while and look around ;)

BECAUSE

Strength without control can be destructive!

Wish you wisdom,
George