The presenter, who is travelling from Edinburgh to London under her own steam with the bulk of the journey by bike, needs to take on 8,000 calories each day to meet her body's needs.
But the 46-year-old - who is cycling 90 miles from Edale in Derbyshire to Birmingham on her latest leg - said she can barely stomach her meals.
McCall said: "You would think I would be so excited to be able to consume 8,000 calories because normally I am a quinoa and lettuce sort of girl out of necessity. But actually when you are training as much as this, and this is something I have learned as well on this challenge, actually I don't want to eat.
"People are putting (in front of me) what would be your absolutely dream meal right now ... and I'm just thinking, I really can't eat - it makes me feel sick. just looking at it. So that's the hard thing. Bit off food."
McCall has won over the public with her resilience over the past few days with TV viewers seeing her sobbing and barely able to enter the water during a 1.5 mile swim across Lake Windermere on Monday morning. Her body was limp by the time she was dragged from the water at the other side and she then had to continue her journey in the saddle for the rest of that day.
Yesterday she rounded off a bike leg with a seven-mile hill run on the Pennine Way and her trainer Professor Greg Whyte advised her to have a lengthy meal alongside her support team in order to make her take on calories over a long period to make the process easier.
"Greg said last night right we're going to sit down with everybody and we're going to have a two hour meal because you will graze and I did eat more last night than I have done the whole event," McCall said.
The former Big Brother host, who is due to reach London on Friday, said she was feeling stronger after being at a low ebb earlier in the week.
"I feel much better than I did yesterday," said McCall, who is aiming to raise hundreds of thousands of pounds with her BT Sport Relief Challenge: Davina - Beyond Breaking Point.
McCall is also finding that all those hours in the saddle are taking their toll.
"Things are hurting a lot. My legs are really sore, my knees are killing, but my undercarriage is desperate! I feel like I'm going to walk like John Wayne for a week," she said.
"I had a lovely message yesterday from a lady telling me she knows just how I feel, like you just want to sit in a puddle - and that's what I want to do.
"I'm trying to find ways of helping, but nothing really works because the damage was done on the first day and I can't give myself time to repair it - it's just ongoing.
She also had to contend with cycling in strong winds today as she made her way to Birmingham's Bullring.