You can find MET charts of different activities in textbooks or in the 2011 Compendium of Physical Activities paper: https://www.ncbi.nlm.nih.gov/pubmed/21681120

You can compare the bicycling equivalent of walking in miles by comparing the total power (in Watt) of biking with the work produced by walking. However, keep in mind that the produced work depends on body mass, while the energy expenditure also depends on individual characteristics such exercise economy, oxygen consumption, etc.

Thanks for your answer Konstantinos, I forgot to add that we are not using a controlled environment and the data collected will be pretty much self reported since this project aims are to encourage participants to engage in outside physical activities

Rightly said by Konstantinos Papaioannou sir. MET chart gives useful information. but keep in mind that body fat and fat free mass play a key role for individual's fitness and energy expenditure along with exercise adaptation.