24AWG for 250 ft is probably ok. One thing you can check is the wire resistance. Use an online calculator to compute the wire resistance, such as this one:
http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
keep in mind that you need to input the round-trip distance. Typical sprinkler valves draw about 200 to 300mA holding current, and if the voltage drops below 18V AC they are unlikely to stay on. So the resistance shouldn’t be more than (24-18)/0.3 = 20ohm, and it should probably be considerably less than that to allow some margin.
Also keep in mind that it’s really just the common wire and the ground return wire that needs to have as little resistance as possible. You can always use two wires in parallel to effective reduce the resistance in half. The ground return wire is part of the extension cable — among the 8 wires of the extension cable, two are connected together as ground, so it should be sufficient. The common wire is not part of the extension cable, and you should try to keep it a thick wire if you can.