riding a heavy bike to burn more calories is a false economy, since if you ride a lighter bike you will just ride it faster, and thus have more fun, in turn therefore being more likely to want to go out again or take a more scenic route.
It's also worth mentioning that there should be no need to use panniers just to carry your cycling stuff, e.g. things you might need duiring the ride like tools, spare tubes, clothes, etc. Shopping, gym kit, stuff you need for work, etc., fair enough, but you shouldn't fall into the rut of thinking you 'need' to use the pannier for every ride just because it's easy to throw all your stuff in it and/or 'cos you can't be bothered to take it off your bike. If you fall into this rut, you'll slide further deeper into it - taking things you are very unlikely to need 'just in case', and worse, not bothering taking stuff out that is no longer even relevant.
Bungeeing things to the pannier rack is however perfectly acceptable and is the recommended way of carrying lunchboxes and/or clothing/waterproofs not currently in use.
It's also worth mentioning that there should be no need to use panniers just to carry your cycling stuff, e.g. things you might need duiring the ride like tools, spare tubes, clothes, etc. Shopping, gym kit, stuff you need for work, etc., fair enough, but you shouldn't fall into the rut of thinking you 'need' to use the pannier for every ride just because it's easy to throw all your stuff in it and/or 'cos you can't be bothered to take it off your bike. If you fall into this rut, you'll slide further deeper into it - taking things you are very unlikely to need 'just in case', and worse, not bothering taking stuff out that is no longer even relevant.
Bungeeing things to the pannier rack is however perfectly acceptable and is the recommended way of carrying lunchboxes and/or clothing/waterproofs not currently in use.