Think M25  - start on the righthand side [East]. If thsi was a clock you would call it 9 o'clock - then go North East up the A12 for 60 minutes.
Specifically, Alton Water is situated just south of Ipswich
The run starts at the Alton Water Visitor Centre.

The entrance is in the village of Stutton  - address Holbrook Rd IP9 2RY.

Directions from London
Reach the M25 by your preferred route.
Go clockwise or anticlockwise to reach Junction 28 - East Anglia via the A12.
Travel North East on the A12.
As you approach Ipswich you reach the A12/ A14 junction  - turn East.
Exit at the next junction for Ipswich Central & Docks.
At the top of the slip road turn left towards Ipswich.
You go down a steep hill past the ski slope on your right.
At the roundaboutot turn right onto The Strand.
You pass Fox's Marina on your left.
Go under the Orwell Bridge and through Freston, You pass The Boot public house on your right.
Take the Right fork to Holbrook & the Royal Hospital School.

Go through Holbrook - The Compasses public house is on your right: go down the hill.
As you come up the other side you pass through accomodation associated with the Royal Hospital School and the school itself is on your left.
Approximately a mile further and the entrance to Alton Water is on your right side

From The North 
M1 - M25 - A12 as above, or A1 - A14 towards Cambridge and thence to Ipswich.