I ran 1:35 last year and hope to do something similar this year. Looking at my times, I just run slightly faster on the flat sections and slightly slower on the uphill sections. But run each successive 5km slightly faster. That seems to work. Good luck.
Lwdjao - I put down 1:40 on the entry form (the time I ran the previous two years) and was in Orange C. I finished, this year, in 1:35 and when I checked my watch found that I'd run an extra 500 metres - dodging around people.