We used miles to upgrade Int'l coach for years, mostly on UA and DL. Over time, the cost of upgradable coach fares went up (a lot), the miles to upgrade went up, getting the upgrade became increasingly hard and, most importantly, C class service went downhill to a point where it's now a bigger seat and luck of the draw. No more for me.

If you want C class and you have no status (and do not plan to act to get status), you'll have to put up miles at the going rate. Minimize the miles you use by choosing the best routes in terms of least miles to get where you want to go. Book way ahead, especially for high season travel, and for anything to do with Delta. Book only at the lowest mile level and work around the limited availability.