Wor Lass says if you twist an ankle, you've got to 'ice it' (a bag of frozen peas will do) as soon as possible after the injury, and take anti-inflamatories.
If it's severe, all swollen and stuff, avoid cross-country runs for four-to-six months; since the tendons will be stretched; the signals your brain receives from your ankles while running over uneven ground could be deceiving, and you could possibly twist the said ankle again; maybe worse.
She also suggests that you're a bit soft; but I don't believe that; but she trains on an injury... so can you!
Just rest it for a day-or-two, then get back to the programme.
(I'm great at giving advice... )