Treadmill running - incline vs flat

Question
What is the relationship between speed and incline?
I've experimented with my treadmill (Trimline 1050-2E) and in terms of calories there appears to be a rough equality between the following:
 10 kph at incline 5,
 12 kph at incline 3, and
 16 kph at incline 1.
Is it ok to run slower on an incline in order to train to run faster on the flat?   
For the record, I'm 51, fairly fit but not really a runner. I'm in training for the UK's Great North Run half-marathon in Sept.
Many thanks
Colin Packer
ps If you have the basic physics formulae to hand then that would also be a help - I've forgotten everything I learned at school!

Answer
Hi Colin
It is ok  to run slower on an incline,but the only thing it will do is strengthen the appropriate musculature which is a good thing,but won't necessarily improve your speed.Hill repeats do have a place in your  preparations,but you have to do speed work at various distances such as 200,400,800,1600,3200 meters.The thing you should focus most on to enhance your performance is your endurance and strength which would be increasing your mileage to level you can handle with 1 long run a week and focus on good hard tempo runs of about 45min.once a week.I hope this gives you some insight for a plan Colin.If you have any other questions just get in touch.

All the very best

John