Duathlons usually appear among the triathlons. A big one is Dambuster, but there are a few around the early triathlon season because it is no fun cycling/running around soaking wet. In the north east Stokesley is this coming weekend, followed by Morpeth on the 9th of April, Durham on 29th of April. See the links in other posts in this section of the forum for triathlon sites.