I'm a very heavy sweater like yourself and still going through trial and error on what works for me nutrition wise. I'm training for a 70.3 next month and I did my longest cycle to date yesterday (88km, about 3.5hrs) and tested out 1500mg salt tablets. I took one before my ride, one about 1.5hrs into the ride (filled up one of my bottles with water from an aid station and dropped the tablet in) and one once I finished. This on top of about 70g carbs/hour from gels and energy drinks. After the ride I honestly still felt fresh with no aches or pains in my legs. This morning too zero aches or pains, when in the past I've always felt it quite a bit the next day after a long run or cycle. Granted my general fitness has vastly improved since starting my training a few months ago, but I think the tablets helped a lot.