Actually, 2 units is not much, but you can never be enough carefully when mixing with insulin. So better start low, and see how you body replies to it. Later you can add it up by 1-2 units. As long as you take it carefully and keep enough of carbs at home, there should be no problems. Remember though, that Humalog is really fast acting, so don't wait to long til you eat/drink. Especially if you take it after the exercise when your bloodsugar already might be low.