As a standard immersion heater is 3KW, so it would take 2 hours to heat 100 Litres of water. After an hour and a half, it will only have reached about 53 degrees.....
Another common cause of heel and calf pain is Achilles Tendinitis. Symptoms are frequently described as an aching pain in the back of the lower leg and just above the heel. This sensation is typically worse after exercise and upon first waking up in the morning.
Inflammation of the plantar fascia can also cause this pain -- normally at the bottom of the heel. Known as plantar fascitis this can cause your symptoms. Both of these occur due to overuse of the foot, often with an abnormal foot arch. Often, shoe inserts or orthotics are needed.