Large range delay at medium frequency for MC
One way to approach the problem would be to introduce some additional variables that keep track of the timer interval and how many times it's been called so that a fixed number of interrupts may be ignored. For example the following C pseudo-code to slow the execution time down by a factor of 10. 

uint_8 ticks_left, timer_interval;

void my_isr()
{
    ticks_left--;
    if (ticks_left == 0)
    {
         disable_timer();
         // Move to next ADC etc
    } else
         setup_timer(timer_interval);
    }
}

void main()
{
    timer_interval = your_calculated_value;
    ticks_left = 10;
    setup_timer(timer_interval);
}


It won't be perfectly accurate because of the (probably less than 10 cycle) instruction delay between the interrupt and the new timer value being set, although if that's a concern you could check the assembler output to find the number of cycles it will take and reset the timer to a slightly lower value in the ISR if that's likely to be a problem.