The second principle of training is specificity. You can learn to walk faster or more efficiently by swimming or cycling. You must do the specific exercise needed to make the specific changes desired. The most common mistake made by beginners in every sport is to go the same distance, the same route every day. This results in no overload and is followed by no rest.
Most athletes think that the hard days are the most important.
To allow the body to benefit from the hard days, the easy days when the rebuilding occurs are often the most valuable. Experience both personal and that gained from coaching others shave shown that 3 total hard days seems to be the max for most athletes. Those trying to get away with more often are injured or sick. Any of the alternatives listed above can be done on easy days. Easy days are always done at an easy pace.