Upgrading from 4.2.6 to 4.6.8
CiviCRM 4.6 officially supports direct upgrades from versions as far back as 4.0.8, so you fall within that window.

If you've already done a trial run on your test site and everything checks out, I'd say you're good to go. Back up your live site just prior to running the upgrade.

If the core upgrade works fine, then the biggest concern about things that could go wrong is with integrations - if your site has any custom code, modules or extensions related to Civi, be sure to review them to ensure they still work with the latest version. I would do thorough testing on your test site first.