Hilly course vs flat

I was wondering... how much do the hills really slow you down?  I did the Watford half marathon yesterday and it was very hilly.  I still managed a pb which I'm very chuffed with but keep on wondering if I could do better on a flatter course.  In a way what goes up must come down so I guess you make up the speed on the downhill so if it's a lap course it shouldn't matter too much?  Or does it as a rule tire you out more when you go up and down?

I'm doing the Madrid marathon in April which is meant to be quite hilly and trying to figure out what I shoud be aiming for... Based on my 10k and half mara times I would say 3h50min but I've been told that because of the hills I should add at least 15min to my predicted time. 

What are your experiences?  Do you find that your times vary significantly depending on how flat/ hilly is the route?

Thanks!

Mon

Yes - I cannot run up hill as fast as on the flat (or not for long, anyway).

Downhills are great if they're gentle, but a steep downhill can slow you down too.

Also, the hard work of trying to run fast up hill tires your legs, so affects your ability to maintain your speed on the flat.

Try putting your Watford time into the Macmillan calculator and see what it predicts.

Most races start and finish in the same place dont they - so theres no height difference.

I think moderately hilly races are just as fast as flat - you slow on the uphill and gain on the downhill.

Really severe races like the Langdale Half though will slow you down. I could barely walk up the hill and as Wilkie says - coming down was just too steep to take advantage.

 With Madrid I think I'd be more worried about the heat than hills ? How warm is Madrid in April ?  

It depends on the year... Last year apparently got up to 26 degrees by the end but other years were cooler.  I'm worried about the heat as well.  I'm worried about lots of things simultaneously.  Others include the altitude (apparently doesn't help either).

Wilkie- mcmillan is telling me that I should be aiming for 3h 45 min for the marathon.  Seems a bit optimistic... The 10k time however is a good minute slower than my 10k pb.  Maybe I should be aiming under 4h after all and worry about the hills when I get to them

Odd first line from Coughie there. Yes, there's no height difference from the exact start to exact finish often...but the other 13miles of the half might have huge variances!

If you can a pb on a hilly course, especially over a half marathon you'll probably be able to take up to 2mins off on a flat course with the equivalent fitness.

However, I've found it doesn't work the other way round.

Did 3 half marathons last year...flat, hilly and flat.

The 2 flat ones were 22 seconds different, and the hilly one was 5 1/2mins slower.

You misunderstand me Shtevie -  overall theres no difference in height - so what you go up - you come down ? 

Cycling time trials have rules about how far away the start can be from the finish - otherwise people would be claiming PB's on a downhill course.  

Right then.  I'm doing the Reading Half (which is meant to be flat-ish) in 5 weeks so based on what you're saying and my times I think it will be reasonable to aim for 1h45min (did 1h46min57s in Watford).  Bring it on!

Thanks for the responses!