Using pruned-enriched Rosenbluth method (PERM) simulations of a discrete wormlike chain model, we provide compelling evidence in support of Odijks prediction of two distinct Odijk regimes for a long wormlike chain confined in a nanochannel. In both cases, the chain of persistence length l(p) is renormalized into a series of deflection segments of characteristic length D(2/3)l(p)(1/3), where D is the channel size. In the first (classic) Odijk regime, these deflection segments are linearly ordered. In the second Odijk regime, thin, long wormlike chains can backfold at a length scale quantified by the global persistence length. We have measured this quantity by simulations and modified Odijks global persistence length theory to account for thermal fluctuations. The global persistence length, which is defined to be independent of the effect of excluded volume, provides the requisite closure to Odijks scaling theory for the second regime and thus allows us to resolve much of the confusion surrounding the so-called transition regime for DNA confined in a nanochannel. We show that Odijks theory for the backfolded regime correctly describes both the average chain extension and the variance about this extension for wormlike chains in channel sizes between the classic Odijk regime and the de Gennes blob regimes, with our data spanning several decades in terms of Odijks scaling parameter xi. Although the backfolded Odijk regime occupies a very narrow range of D/l(p), it is indeed a regime when viewed in terms of ? and grows in size with increasing monomer anisotropy.