Is 2 hours of walking a week enough to keep you fit?

​That is a good question.
It depends really. Walking in your comfort zone meaning you are walking at a pace where you can comfortably hold a conversation, referred to as zone 3. This is better than no activity but it is nowhere near the complete fitness package. If you walked at a higher pace at times and get into zone 2, This is where you can hold a conversation but your sentences might be breaking up a bit.If you can try and stay in zone 2 for a few minutes during your walk you will have many more benefits. There is also a zone 1.This is when you push really hard perhaps when you walk up a hill and you get to a point where you have to stop or slow down.
So if you go for a 45 minute walk and you want to make the most of your time, warm up in zone 3 the increase the pace to level 2 and stay there for a prolonged time. If you want really make the most of your time try and push really hard when you walk up a hill and get to a point where you want to stop.
The national guidelines for exercise is that you spend between 3-5 hours doing some movement every week and on two occasions you should involve some weight bearing exercises.

 So two hours of walking at a comfortable pace is not really enough by any means.

 The weight bearing exercises should focus on the biggest muscle groups, the legs, the upper back and chest.
The minimum amount of sets you need to do to get stronger is 25 sets per week. This could be 10 sets of legs, 10 sets of upper back and 5 sets of chest.

 If you get into a routine of both cardio and weight bearing exercises you could avoid many life style diseases and life a happier and more fulfilling life as you get into your senior years.