Salman:
Normally for optimal results you want to use at least a 20% gradient hill for the weighted pack carries. If you calves are getting very sore that does indicate muscular weakness and you might consider doing weighted calf raises for a few weeks to build some strength there. For those living in flat places we recommend the use of stair machines for these extended pack carries. They are steep and the workout will be continuous without going up and down. Much more time efficient.
Good luck,
Scott