That would certainly be about a $100 job at home, but the convenience factor on the road is worth the money. You do not want to overheat, ever.
An '09 with a water pump issue seems early to me. I have an '06 with 109K miles, and I am still on the original pump. How many miles were on your OEM pump?
I had an issue on the XK last year where the overflow tank went empty. I was 900 miles from home also. My issue was the 3-way splitter on the hotside for the heater hose sending hot coolant back to the third row seating HVAC controls. It sprung a leak and sprayed out. I spent about $30 at Napa on a new generic splitter and some HOAT coolant and fixed it with a pocket knife and a screw driver in the in-law's driveway. I had to drive 900 miles the next day. My patch is still solid a year later.
About 800 miles into that trip, I had a previous tire patch repair blow out. I got to put on the spare before getting home. You have got to love the full size spare. That was a Murphy's Law trip. All is well that ends well.
'06 XK Limited, 5.7L Hemi, Quadra-Drive II, Black, 114K Miles
'02 WJ Overland, 4.7L HO, Quadra-Drive, Inferno Red, 96K Miles