hi martin

at 12yrs old they should be fit, but ive always told my lads if they want to be fitter then they need to do the work outside of training.

only having an hour or two maybe a week to coach.

i would just concentrate on high intensity SSG's 

ive found its the mind that gets tired more than the body with the players at that age

Play games with loads of balls ready to throw in whenever the ball goes out. This can make a full sided game more demanding but makes small sided games into full on fitness sessions. 

Timing the periods and keeping score will keep the intensity high and allow the players to recover in 1-2 minutes then go again.

Hi Martin. Have a look at sessions on the England Learning that incorporates an element of fitness along with ball work whether that be in the in possession phase or out of possession. Maybe consider fun competitive games that ensure they can focus on the fun whilst working on their fitness

Hi Martin, In terms of fitness for under 13's I agree with other comments that recommend lots of small sided games and high activity levels or ball rolling time in sessions. Some things that can support high ball rolling time are arrival activities so the players are active as soon as they arrive, working in smaller groups, smooth transitions between practices and limited interventions from the coach. A wider consideration for u13 players related to fitness training is that they will be going through adolescent growth spurt at different times so generic fitness activities will have limited benefits until they are post puberty. Instead the recommendation is to focus on football activities which provide fitness benefits as a by-product.