I’m a 68 year old Long Distance Runner , have ran 1/2 marathons and 10k’s !! After I become aerobic while running, I do fine , But until Aerobic stage , it’s a tough run as I have to walk as I’m out of breath! My question is how can I run a 1/2 and can a

Hello Anon

Thank you for writing and well done for staying so fit.

The aerobic stage of exercise is when there is plenty of oxygen in the body and the body is stable, thus running and breathing becomes steady.
I have the same problem when I use an exercise machine (not a kind of exercise I like but winter sometimes insists on it). I have to take multiple minute long breaks to get more 02 in my body. Then finally I get to the aerobic stage.
It's ok to walk it out during this stage, you need to listen to your body.
Since you are an experienced athlete, there may not be much more you can do stop this method of getting through your races.
It's interesting that you get winded by going up a stairs - but not unusual! There is an article on Health.com called "Here’s Why You Get Out of Breath Walking Up Stairs (Even Though You’re Fit)" which is exactly what's going on with you, I think it'll help you a lot! Cut and paste here:
http://www.health.com/fitness/winded-walking-up-stairs
Let me know what you think!
Susan