First, when we speak of energy changes for a reaction, this can mean many things but usually it refers to changes in (Gibbs) free energy, which involve both entropic and enthalpic differences between reactants and products. Heat consumption or release primarily refers to the enthalpic portion. The "context" I alluded to was your mention of using bond energies. This is also a viable way of estimating heat loss/gain by a reaction, but it is more cumbersome and less precise in this case because bond energies do not take into consideration of intermolecular interactions, which are incorporated in widely-available heat of formation data.
Second, the way you determined the enthalpy change is correct. However do note that your equation is not balanced for charge and therefore your decomposition reaction is not realistic. Nitrates are anions in aqueous solution and always have a counterion which needs to be incorporated into the equation in some way.
A more realistic equation might be something as follows:
4NO
3-(aq)
2O
2-(aq) + 4NO
2(g) + O
2(g)
Or some such...
(Likely the superoxide will be bound to one of your metallic cations and precipitate as a solid; either that or it will disproportionate with water into hydroxide and oxygen gas, so even the way I have formulated it here is probably unrealistic, but I think you get the idea...)