You can go with CentOS 7, which will be supported for ten years. You can install LXDE/LXQT on it. The other option is Debian 7, but for now it's not clear for how long it's going to be supported.
Long support extends the period of software maintenance; it also alters the type and frequency of software updates (patches) to reduce the risk, expense, and disruption of software deployment, while promoting the dependability of the software. This mostly applies to security updates. While in theory developers could backport just the security fixes from each new release to their deployed version, in practise this would often be prohibitively difficult or costly. There is a moment when backporting costs more than new deployment. So, if you really want a stable release for a very long time, go with servers distros like debian or centos - in enterprise, deployment of new version is very expensive, and because of that support is longer. And for regular user proportion of cost/risks is lower, that's why support is shorter.