They are long, steady climbs and the best strategy is to stick to a comfortable work rate, sit back and gear down so you can spin.
Your MTBs will have plenty of low-end gearing but you may want to use slick road tyres for lightness and efficiency. 1.5" slicks can still handle riding along tracks and trails.
I found that some MTBs are setup with the bars too high for climbing. A cross-country setup with the bars and inch or so lower than the saddle is often more comfortable.