I've heard that while adder bites are poisonous the number of people who have died from them are extremely low. If you're on a trek and you're bitten is it safe to continue for a couple more days before seeing a doctor or should you still abort right away and seek medical attention?
Part of the reason that people don't die from snake bites is because they take the proper precautions. And while the bite of an adder isn't likely to kill you, it could make your return trip much more difficult.
One thing you do not want to do is to try and suck the venom out, or try and wrap a tourniquet around it. These will stop your bodies native ability to get rid of the venom.
Treat the bite to prevent it from bleeding, and then go as quickly as possible to a qualified medical facility, without straining too much.
If you are more than 24 hours away from help, then you might want to simply get to a good area to camp, and relax during that time. But try and find an area where you can request medical help if needed (IE, has cell phone reception). You probably have a few hours of hiking before too many symptoms would appear.