I'm assuming you have the Kenda Kwest? IMHO these are a great tyre - one of the unsung heroes of 20" tyres. There is a high pressure (100psi) version available. I have had one of mine have a carcass failure, it developed a bulge which got worse over some days. The Kwest has quite OK rolling resistance and good grip.
You should be able to get far over 5000mi, perhaps 10,000mi on the back. The front you will probably not have to replace unless the carcass fails.
Don't rotate the tyres, just replace the back one when the canvas shows through or you begin to get too many punctures. You could opt to get a Big Apple for the rear; that is another great tyre which rolls well when pumped medium while affording a nice degree of suspension.
While the kendas don't have the best performance, it won't show much of a difference if you put 'faster' ones on.