What would probably happen if a long neuron had one continuous myelin sheath down the length of the axon with no nodes of ranvier?

slow action  potential  will be generated

Explanation: Nodes of Ranvier are interruptions of myelin sheath/gaps along the axon. The nodes contains more ion channels than any other parts of the axon.

Myelin sheath are lipid components of the axon that insulated the neurons, and maintains integrity of the action potential and nervous transmission along its length.

These gaps increase the speed  of action potentisl by making the impulse to jump when it gets to the nodes with more ions channels, and therefore more depolarization. This increases the speed of transmission of action potential along the axon. This is call saltatory conduction

However ,in absence of nodes in unmyelinated neuron, the nervous transmission will be slowed down; because few ion channels of sodium and potassium are available for depolarization along the axon lenght.Therfore no jumping or saltatory conduction.