If you are losing 50%, your chance of losing 10 in a row is:
1/(.5 * .5 * .5 * .5 * .5 * .5 * .5 * .5 * .5 * .5) = 1 in 1024.

If you are winning 20% and losing 80%, your chances of losing 10 in a row is:
1/(.8 * .8 * .8 * .8 * .8 * .8 * .8 * .8 * .8 * .8) = 1 in 9.3

You cannot workout a longest losing run, because there is no such thing. Whilst there is a chance of losing the next race, the figure is infinate.