What are the health impacts of standing / sitting while lecturing?
The health impacts of standing while doing a job are known to be strongly positive, compared to sitting while performing a similar function. For the seminal paper on this, see Morris et al., 1953 in the Lancet (http://www.sciencedirect.com/science/article/pii/S0140673653914950). Morris examined the incidence of heart disease in bus drivers vs. bus conductors. The former spend the day sat at the wheel while the latter spend the day walking around the bus collecting fares and enforcing the rules. This occupation was chosen because drivers and conductors come from the same socio-economic conditions and were expected to be fairly similar in every respect except for their working activity. Naturally the incidence was sharply reduced in the conductors. A casual Google Scholar search will pick up more recent references, and the health benefits of standing vs. sitting are widespread.

One can imagine that standing for very long periods of time repeatedly might cause problems, particularly on the feet or ankles, but seven hours a day a few times a week is not really anything to be excited about. Many people work in bars or restaurants, on building sites, in hospitals and so on doing much more labour intensive work with shift lengths exceeding twelve hours, six days a week.

All I would say is ensure you wear comfortable shoes!